1. Field of the Invention
The field of application of present invention relates to weighing systems of vehicles in motion also called “WIM systems” (Weigh In Motion).
2. Brief Description of the Prior Art
The weight of the vehicles is a very interesting piece of information in many applications and its knowledge is of great importance.
The most common methods for the weighing of vehicles consist in a static weighing: the vehicle stands above a weighing scale, it is stationary, and the weighing scale, which dimensions are sufficiently large, provides a weight measure.
It is clear that the static weighing procedures are processes that, although they can achieve high accuracy, they cannot be applied in all contexts in which it would be necessary to know the weight of a vehicle. The main limitations are in the slowness of the procedure and in the fact that the “cooperation” by the driver of the vehicle is necessary, since he must go to position his vehicle precisely above the weighing scale. It is clear that applications for the identification of vehicles that circulate overweight, or applications aiming to calculate the amount of a toll according to the weight of the vehicle, are all applications for which the methods based on static weighing are not suitable.
The known art, therefore, also offers other weighing methods which allow to determine the weight of a moving vehicle to its passage over an appropriate dynamic weighing system. Such systems, commonly called “WIM systems”, have the major drawback that do not reach as high precision as static weighing systems and, above all, they have operating limits defined by a certain maximum speed of transit.
Said WIM systems would have a wider application if it were possible to increase the accuracy and the maximum speed of transit within which the measurement of weight is reliable.
Some known WIM systems are made by positioning a metallic plate at the street level and vehicles to be weighed pass above that. Such a metallic plate, also called “loading plate”, is mounted above a cavity obtained on the road surface. In this way, said cavity is covered so as to offer a substantially continuous plane formed by the road surface and the upper surface of said loading plate over which the vehicles transit.
When a vehicle passes over said loading plate, the latter deforms being able to bend toward the underlying cavity. The deformation of the plate is evidently more accentuated the more heavy is the vehicle that passes over it.
There are several ways to measure the deformation of the plate, however, it is not easy and immediate to correlate the weight of the vehicle that has passed over the plate to the deformation undergone by the plate itself because there are many variables that determine said deformation. Hence, substantially, the difficulty for these WIM systems to achieve high measurement accuracy.
A very efficient way to obtain a measure of the deformation of any object, whereby also the deformation of a metallic plate, is to use a FBG sensor (Fiber Bragg Grating).
In short, said FBG sensors exploit a property of the optical fibers that can be processed so as to form a segment that internally behaves as a “Bragg grating”. A fiber behaving as a “Bragg grating” can be used as a sensor of deformation; in fact a “Bragg grating” has the property of reflecting, very selectively, a particular wavelength, when it is invested by a broadband radiation. If, however, the fiber with the “Bragg grating” is deformed, the “Bragg grating” obtained inside said fiber is deformed accordingly and, as a result, the frequency of reflection of the grating itself is varied too.
Thus, a FBG sensor is essentially a segment of optical fiber processed so as to behave as a “Bragg grating”: in fact transmitting a broadband optical signal over an optical fiber with a “Bragg grating”, and measuring the frequency of reflection, a measurement related to the deformation of the fiber itself can be obtain.
Ultimately, associating to a body that can be deformed, some FBG sensors on which it is possible to transmit a broadband optical signal and performing the measurements of the wavelength that is reflected by the FBG sensor, it is possible to derive a FBG-signal which is related to the deformation undergone by the body at the point where sensor FBG is applied.
Thanks to said FBG sensors that can be applied in several points of a metallic plate used to make a WIM system, it is possible to derive a multiple FBG-signal composed of a set of elementary signals. Each of said elementary signals is a function of time. The number of elementary signals corresponds to the number of sensors associated to said metallic plate and the variability over time depends on the fact that a vehicle takes time to pass above said metallic plate by producing a deformation varying in time. Said multiple FBG-signal is related to the deformation of the plate and thus provides a measure of deformation.
In addition to the FBG sensors, other technologies allow to obtain a signal related to the physical deformation of a solid body. The application of FBG sensors is shown, in this case, as the preferred solution and, in the following, the invention presented in this description will often make reference to said FBG sensors; however, any sensor able to provide a signal related to the deformation of a loading plate can be used for the implementation of the concepts taught in the present invention.
In conclusion, we can state that the deformation undergone by a loading plate, when a vehicle passes there over, can be measured.
In order to synthetize a numeric measure of the weight of a vehicle that, passing above said metallic loading plate, deforms it, it is necessary to use numerical models that describe the deformation of said plate as a function of applied stress. Moreover, those models should be as accurate as possible in order to go back to a sufficiently precise measure of weight.
Therefore, there is a technical problem in providing a WIM system in which the mathematical model that describes the deformation of a plate, caused by the stresses produced by the transit of a vehicle above it, is the most simple and corresponding to reality.
In order to model the deformation of the loading plate in an accurate and realistic way, it is convenient that said loading plate is fixed to the ground by means of a simple support on the edges of the plate itself, and the deformations are just caused by weight forces applied at intermediate points between the supports.
In addition to the easiest mathematical modeling, the simple support presents additional advantages in that the resulting FBG-signals, for effect of the deformations of the plate, have a more simple and less noisy shape. Furthermore, it can be conceived a system whose overall deformations, occurring during the weighing processes, have repeatable characteristics which remain uniform over time.
Other types of constraint (for example with pinouts burdened by the weight of the vehicles or hinges of various types) are certainly more subject to wear out and they would compromise the repeatability of the measurements with the required accuracy.
The simple support presents the obvious limit that it is a constraint only in respect of vertical stress; therefore, it is necessary that also the horizontal forces are compensated as much as possible avoiding abrupt horizontal displacements and bumps, due to the locking of the plate, along horizontal directions.